I have read 100s and 100s of posts related to rolling diameters but am still confused about something. I hope someone can help me out here.

So I have an '08 E92 xi stock setup with 225/45 17X8 all four wheels.
I want to upgrade to: 225/40 19X8 front; and 255/35 19X9.5 rear.

So my issue is -- I have seen people talk about rolling diameter in two different manners:
1) the % difference between individual wheels
2) the % difference between front wheels and rear wheels

Individually, my rolling diameter % difference is about ~4-5%, which I know will impact my speedometer by about 3 mi/hr. Which is fine with me. But does it mean that it may ruin my ABS/AWD system? The confusion is that, when people talk about the ABS/AWD + upgrading wheels, they talk about the overall % difference (front vs. rear).

So, front vs. rear with the new setup, my % difference will be 0.22%, which is less than the recommended 1%. Am I correct? Also, my front vs. rear RPM diff will be 3, also within suggested specs by some users.

I just want to verify WHICH rolling diameter difference is what matters with respect to ABS and AWD systems -- the individual old tire v. new tire, or as a package -- front tires vs. rear tires?

How in the world do you get 4-5% difference? Strictly speaking the diff should be less than 1% under any circumstances F/R or side/side or any combo thereof due to original size or wear.

IIRC (I looked this up in the owners manual once a long time ago) they say replace tires on a given axle in pairs if one goes flat an the other is worn to more than 1% difference of the new one. I am sure someone will jump in and say I am full of shit but I do remember something like that in there.

I meant IF I go from:
Current: 225/45 17X8 (all 4)
To: 225/40 19X8 F and 255/35 19X9.5 R
then, the rolling diameter would increase from 4-5% of each wheel. But the difference in rolling diameters between the front set and the rear set for the new staggered set would be 0.2%.

I am still confused as to how some people have put this setup on their E92 xi as I have seen people do it :S

Oh, I see. It does not matter that the old set of tires size vs the new set of tires' size is different. It only matters < 1% within a given set on the car at the same time.

edit: the reason why is you do not want the tires/wheels rolling at different relative rates. Because if you think about it it is all metal fluid and clutches in there and something has to slip/give. Also with ABS, the operating principle works off of sensing different rates of wheel rotation front back or side side, and relieving braking pressure on one wheel (stops it skidding) so it catches up to the other. If the sizes are differnt that screws up the sensors reading of the situation and activation thresholds. Same applies to sending power F/R or side side with the Xdrive system.

So there are two issues, one is excessive wear on hard parts, the other is functional interference.

Oh, I see. It does not matter that the old set of tires size vs the new set of tires' size is different. It only matters < 1% within a given set on the car at the same time.

edit: the reason why is you do not want the tires/wheels rolling at different relative rates. Because if you think about it it is all metal fluid and clutches in there and something has to slip/give. Also with ABS, the operating principle works off of sensing different rates of wheel rotation front back or side side, and relieving braking pressure on one wheel (stops it skidding) so it catches up to the other. If the sizes are differnt that screws up the sensors reading of the situation and activation thresholds. Same applies to sending power F/R or side side with the Xdrive system.

So there are two issues, one is excessive wear on hard parts, the other is functional interference.

Great explanation and it makes perfect sense. My intended setup only has 0.2% of rolling diameter difference, so I should be good then

Yes, that is the reason I started this thread I bought a set of wheel (style 67 from the e46 M3) and tires (ContiSportPro) used, which came with 19s -- 40 and 35. And since I see almost everyone on 35s and 30s, I was wondering how would the slightly larger sidewalls affect my car other than looking meaty + having a slight speedo difference. Obviously my main concern being the AWD system.

But if Speedo difference is the only issue, I would not personally mind as Chicago roads are just merciless to rims. Some extra padding with the tires would not hurt! Also, since mine is an xi, it is slightly higher than the regular i. And since I am not 100% sure if I will lower it, maybe meatier tires would actually fill up some up that awful gap as well.

With AWD, the only thing you need to worry about is that the size of all 4 tires have a difference of no more than 1%. As you stated, the larger rolling diameter will only affect your speedo and tach, not the AWD.

If you do go with those sizes, be sure to throw up some pics. I'd be very interested in seeing how it looks on a stock height XI.

The offsets are front: ET41, rear: ET27. Seen a couple of people have them on e92s without problems, esp. the 19s because they clear the calipers pretty well (I think the 18s don't clear the calipers in the front). I have a staggered set (8/9.5) and because of the offsets, the front may look a bit tucked in, so I will have to see if I would need some spacers, maybe 10-15mm or something in that order.

Always like those wheels, esp the 19" forged versions. Not crazy about the 9.5" on the front in those pics but your staggered set should look awesome esp with fatter tires on the Xi. Be sure to post some pics of yours when you get them on.